The invention relates to a method, a device for measuring optical properties of at least two mutually spaced regions in the case of a transparent and/or diffusive object, and to their use for thickness, distance and/or profile measurement undertaken by means of short coherence reflectometry.
The transparency of objects is a function of their wavelength-dependent attenuation coefficient xcex1 [cmxe2x88x921] and their thickness as well as the prescribed measuring distance d. Objects noted as transparent are those whose transmission factor T=exp(xe2x88x92xcex1xc2x7d) still lies within the measuring range of the interferometers described below, the transmission being T2 in the interferometers described below because of the xe2x80x9cround-trip pathxe2x80x9d of the radiation. In diffusive objects, the radiation is strongly scattered but not necessary absorbed. Milk glass plates, Delrin, organic tissue (skin, human and animal organs, plant parts etc.), for example, are to be regarded as diffusive objects.
Short coherence reflectometry has been used as a rule for precise, quick and non-invasive images. It has been typical for the beam of a radiation source to be split with a beam splitter in an optical arrangement with a Michelson interferometer into a reference and a measuring beam. As a rule, a radiation source with a short coherence length has been selected. Splitting into reference and measuring beam and reuniting them have been performed with a beam splitter and when fiberoptic paths are used with a fiber coupler. It has been possible to achieve the change in the optical path length in the reference arm by displacing a reference mirror on a translation stage. However, use has advantageously been made of a rotating transparent cube such as described in WO 96/35100. It is only when the path length difference is smaller than the coherence length of the radiation of the radiation source that the reflected reference and measuring beams have been recombined to form an interference pattern. The interference pattern has been brought onto a photodetector which has measured the intensity of radiation during the variation in the mirror position. Since the frequency of the radiation of the reflected reference beam has experienced a double shift owing to the mirror displacement, it has been possible for the interference signal to be evaluated with electronic means, as described in WO 99/22198, for example, by increasing the signal-to-noise ratio.
However, measuring errors have occurred when distances which have necessitated at least two measuring operations were to be measured in optically transparent objects, or objects transmitting an optical radiation diffusively, and the objects could be fixed within the required measuring tolerance over the entire measuring cycle only with difficulty or not adequately. These problems have arisen, in particular, in the case of in-vivo measurements.
It is an object of the invention to present a method and to create a device with the aid of which method and/or device it is possible, in particular, to carry out optimally, that is to say with the minimization of measuring errors, in the case of in-vivo measurements of distances, thicknesses, surface profiles, which include measurements at different locations in an object.
The object is achieved by virtue of the fact that in order to measure optical properties using a measuring time in the subsecond range (necessary for an in-vivo measurement) of at least two mutually spaced regions in the case of a transparent and/or diffusive object, as necessary for measuring distance, length, thickness and profile, the object is irradiated simultaneously or in rapid sequence, with the aid of a number of measuring beams corresponding to the number of regions. The expression xe2x80x9cin the case ofxe2x80x9d an object is intended to express that the regions can be located at the locations in the object as well as on the object, for example laterally offset. In each case two measuring beams have, except for a mutual determination tolerance, an optical path difference, that is to say a transit time difference. The transit time difference corresponds to an optical distance between two points in space (regions) with reference to the direction of propagation of the measuring radiation, at least one of the points in space being reflected at least slightly (typically at least 10xe2x88x924% of the intensity of radiation). The measuring beams can thus be situated one above another (thickness, distance, length measurement), and parallel to one another (surface profile, . . . ) or exhibit any desired angle to one another (thickness, distance measurement, . . . in the case of a given angle to a reference surface). Each reflection beam of the measuring beams which is reflected by one of the regions is detected by superimposing on it in an interfering fashion a third beam having a temporal, preferably periodic, variation in path length.
After the path difference or differences has/have been taken, the measuring beams are combined, preferably for thickness measurement, to form a single beam configuration with a single optical axis. Again, the beam configuration can be moved, in particular periodically, over the object. This gives rise to lateral scanning. In conjunction with storage of the values determined, this scanning can lead to the creation of profiles. Instead of the two measuring beams being focused on an optical axis, however, it is also possible in each case for at least two measuring beams to run and be focused at a distance next to one another in order to determine a surface profile.
By comparison with the distances between regions, in particular with the distances between regions starting from a reference location, the measuring beams have a short coherence length. The measuring beams can, furthermore, in each case have mutually differing radiation frequencies. However, it is then necessary to use a plurality of radiation sources. It is also possible to work with only one radiation source and perform splitting via filters. However, this results in a loss of broadband capability; again, some of the components must be provided with an expensive coating.
Instead of different radiation frequencies, or as a supplement to this, the measuring beams can have mutually differing polarization states, which yields a simpler design. Focusing the measuring beams in the region or regions to be measured will also preferably be undertaken. Since the operation is carried out with an optical arrangement of the Michelson interferometer type, the instantaneous position of the reflecting element in the reference arm can serve as reference location. It is now possible for this purpose to use the actual position or another value coupled to the reference location such as, for example, the rotary position of the rotating cube described in WO 96/35100.
The measurement is carried out with an optical arrangement, of the Michelson interferometer type, into whose measuring arm the optically transparent and/or diffusive object can be introduced. Instead of an optically transparent and/or diffusive object, it is also possible to work with an object whose surface is reflecting, In the case of a reflecting object, it is possible to use the method according to the invention to determine its surface profile, in particular. However, the object can be optically transparent and/or diffusive, and have a surface which reflects (at least a few percent). It is then possible in this case to determine both surfaces and thicknesses and/or their profiles. The reference arm has a path length variation unit with the aid of which it is preferably possible to carry out a periodic change in path length in the reference arm. Arranged in the measuring arm upstream of the object is a detour unit with the aid of which at least one first measuring beam can be imparted a transit time which is longer than at least one second measuring beam, it being possible, except for a determination tolerance, to select a detour, which can be produced by the detour unit, to be equal to a distance between at least two regions to be measured in the object. In addition to regions (locations) for thickness measurement which are situated xe2x80x9cone behind anotherxe2x80x9d in the object, it is, of course, also possible to measure regions (locations) situated xe2x80x9cnext to one anotherxe2x80x9d to determine surface curvatures and/or surface profiles.
The detour is set approximately in such a way that it corresponds to an expected measurement result of a thickness, distance, . . . to be determined, except for a determination tolerance. All that is then required is to use the path variation unit in the reference arm to determine the unknown fraction (to be determined) of the thickness, distance etc. If, for example, the actual length of a human eye is to be determined, it is, after all, known in advance that eyes have an optical length of 34 mm with a length tolerance of +/xe2x88x924 mm. It is possible here to set a detour of 34 mm and to use the path variation unit to undertake a variation of only 8 mm.
In addition to the eye length, the device described below, and its design variants, can be used to measure the thickness of the cornea, the depth of the anterior chamber, the lens thickness and the depth of the vitreous humor, as well as corresponding surface profiles of the eye. For this purpose, the measuring beam specified for the surface of the eye as object surface is focused xe2x80x9csomewherexe2x80x9d between the front of the cornea and the rear of the lens. It is then possible by means of this xe2x80x9ccompromisexe2x80x9d to detect the reflection at the front of the cornea, the rear of the cornea, the front of the lens and the rear of the lens. The distance between the rear of the cornea and the front of the lens is then the depth of the anterior chamber. However, it is a condition for this measurement that the xe2x80x9copticalxe2x80x9d stroke (approximately 8 mm) of the path variation unit is so large that it is possible to scan from the front of the cornea up to the rear of the lens.
A single measurement thus processes the reflections from several regions together nearly simultaneously. In order, nevertheless, to be able to distinguish individual reflections by measurement, the measuring beams have different optical properties such as a different direction of polarization, a different wavelength, . . . However, it is also possible to work with non-distinguishable beams and to bring the two interference signals into congruence by varying the detour, In this case, the detour set is then equal to the desired distance, thickness etc. The use of non-distinguishable beams leads to a loss in sensitivity.
Depending on the number of measuring beams used, it is possible to determine one or more distances with one measurement. As described in WO 96/35100, the changes in path length in the reference arm can be undertaken with a rotating transparent cube upstream of a stationary reflector. Such a cube can rotate without difficulty at over 10 Hz. That is to say, in the case of most measurements the object to be measured can be regarded as being at rest without providing special arrangements for fixing it.
The detour is produced with a detour unit in which the geometrical-optical length of the detour can be varied with reference to one of the beam splitters by adjusting the distance of a deflecting mirror. The beam splitter and each deflecting mirror assigned to the latter are, in particular, aligned relative to one another in such a way that each deflected measuring beam with the non-deflected one has a single optical axis inside the object and, optionally, one focusing unit each for each measuring beam, in order to be able to focus the latter onto one region each. Such an arrangement can be used to measure thickness, length and/or distance, and/or to measure the thickness, length and/or distance profile.
It is preferred to make use in the device of a memory unit in which it is possible to store path lengths of the path length variation unit, in the case of which path lengths it is possible to store an interference of the first and third as well as of the second and third measuring beams. The thickness, length and/or distance value will then be determined in conjunction with first and second measuring beams preferably approximately focused on one axis, and/or a surface profile will optionally be determined from the stored data in conjunction with first and second beams situated laterally in a neighboring fashion.
Further design variations relating to the invention, and their advantages emerge from the following text. It may be remarked in general that the following optical units designated as beam splitters can undertake beam splitting, or else a combination of two beams.